Re: [PATCH] improve folding of expressions that move a single bit around

[Marc Glisse]

On Sat, 26 Nov 2016, Paolo Bonzini wrote:

--- match.pd    (revision 242742)
+++ match.pd    (working copy)
@@ -2554,6 +2554,19 @@
  (cmp (bit_and@2 @0 integer_pow2p@1) @1)
  (icmp @2 { build_zero_cst (TREE_TYPE (@0)); })))
+/* If we have (A & C) != 0 ? D : 0 where C and D are powers of 2,
+   convert this into a shift of (A & C).  */
+(simplify
+ (cond
+  (ne (bit_and@2 @0 integer_pow2p@1) integer_zerop)
+  integer_pow2p@3 integer_zerop)
+ (with {
+    int shift = wi::exact_log2 (@3) - wi::exact_log2 (@1);
+  }
+  (if (shift > 0)
+   (lshift (convert @2) { build_int_cst (integer_type_node, shift); })
+   (convert (rshift @2 { build_int_cst (integer_type_node, -shift); })))))

What happens if @1 is the sign bit, in a signed type? Do we get an 
arithmetic shift right?

--
Marc Glisse